2-LEVEL-SYSTEM-RELATED ZERO-BIAS ANOMALY IN POINT-CONTACT SPECTRA

In point-contact spectroscopic measurements on mechanically controllable break (MCB) junctions of several metals, two distinct anomalies are observed near zero bias. The first anomaly is well described by the elastic scattering of electrons from two-level systems with different scattering cross sections for both levels, but may also be explained by another model which involves the interaction between a two-level system and an electronic screening cloud. The experiments suggest that these two-level systems are related to dislocations, due to stretching of the sample at the surface near the contact. The second type of anomaly has an S shape and was observed at higher voltages. This anomaly is found to be remarkably independent of the sample material. The origin of this anomaly is not yet clear. © 1995 The American Physical Society.